低压铸造和高压铸造的比较(英文版)

LPDC Casting process.
The low pressure casting process is similar to the traditional aluminum die casting process. Which
is commonly used to make cylinder heads wheel rims, and the sp. parts which requires pressure tight Function requirement.
Low pressure casting involves the injection of the molten metal with a low pressure and low velocity. Following are the parameters that you forced to produce the component on the low pressure system.
Sr. no. High pressure process Low pressure process
1] High cavity pressure is used to fill the Low pressure is used to fill the cavity casting [ From 600kg/cm.sq. to 1100] [0.2 to 0.5 kg/ cm. Sq.]
2] High gate velocity Low gate velocity
[ 30 to 60 m/ sec.] [ 0.15 to 0.6 m/ sec.]
3] Less filling time Much higher filling time as per the
30 to 100 mili sec. Volume of the casting.
4] No isolated coating is required. As the Isolating coating is used as the filling Solidification
time is less.
Only
releasing Time is more than HPDC.
Agent is
used.
5] High silicon and & non heat treatable Low silicon and heat treatable
Material is used. For casting. Material is used. For casting.
6] Turbulent free flow is not so profitable as Turbulent free flow is require as far as The atomization of the flow is required. possible
7] In HPDC mold cavity air can thrown out In LPDC the aim is to drive the mold 100 %.it is compressed at some except. Cavity air out with the metal flow.
8] High rate of production. Low rate of production.
9] High degree of accuracy. Wide tolerance was used.
10] Low foundry rejection as parameters Rej, is more than HPDC as all the having no successive setting. parameters are very sensitive.
11] HPDC part can be produced on LPDC LPDC Part can not be produced on process. Hpdc due core inout is not possible.
Design consideration.
a] One should see that the feeding wall thickness that is the wall thick near the gate portion should be more [ at least 1.5 times than the other part thickness.]
b] Before ensuring the gate position, the solidification behavior within the part is to be seen.
The part should be solidify in such a way that it will start away from the gate and advances
towards the gate, without enclosing the any pocket or area having the molten metal or
the high temperature of the metal. In this case it is very difficult to come out from the shri-
nkage porosity in the casting at that portion.
So the heavier section of the part should be toward the gate side.so that the temperature
of the die should gradually increase towards the gate. And this is the requirement of the
LPDC die casting process.
c] Heavy wall thickness variation should be avoided.If so change the part design to some extent to avoid further defects.
d] Thin wall [ less than 2mm] should be avoided within the casting cavity.
e] Ensure the direction of the solidification. That is the heat should be extracted from the die part which are away from the gate. So the heavy sect of the die parts which are away from
gate should be avoided.
f] Pilot cooling should be considered around the heavy section of the gate, But the cooling cycle should start when entire casting got solidified. This will increase the productivity as
well as the quality of the casting.The timing of the cooling cycle is very important as the
shrinkage porosity will be left in the unsolidified portion of the casting. Isolating the sprue
and connector this type of porosity can be avoided.
g] Designing the die and sprue bush the following points should be considered.
Sliding part of the die should not have the heavy mass. Heat deception should be more
from the sliding part.
h] Venting position gate position should be marked out
Area of the gate to be decided in such way that the Gate speed within a limit of 0.2 to 0.5 m
per second velocity.
Material melting process.
a] In LPDC casting process the material chemistry plays the major roll of the casting process.
The aim of the melting procedure of the metal is to optimize and to reach a certain level of
mechanical and the theoretical properties of metal which we have used. It mainly influences
the solidification behavior to get the require microstructure, improvement in filling property,
and it will reduces the degree of the known defects.
b] The pouring temperature of the metal is varying as per the silicon and the alloying elements.
But as per the metals used in LPDC casting the temperature should 720 degree to 750
degree centigrade. More the temperature more the oxidation formation. And this oxide incr
eases the casting defects. To avoid that the melting temperature should not be more by
30 degree than the pouring temperature.
c] Grain refinement Or Titan Boron treatment
Grain refining is not necessary for the eutectic Al-Si alloy. But this treatment is specially
effective for the non eutectic alloy.That means the silicon having the range from 5 to 10%
require this treatment to improve the microstructure, mechanical properties, and reduces the
shrinkage defects.
When the undereutectic material solidifies - during the solidification in the alpha phase of Al
alloy dendrites forms.These dendrites grows until the eutectic temperature reaches [ That is
the solidification temperature of the Al alloy having 12% Silicon.] Then the rest of material
solidifies as an eutectic material.
In the solidification process the Ti-B Particles play the role of nuclei form and then it very easy
for the Al dendrites to grow faster. In this phenomena that the higher the number of nuclei, The structure is finer. As the more number of dendrites, with fine in shape and size and staggering
Uniformly gives a good quality of primary structure, and mechanical property.
The following benefits of this process.
1] Feeding behavior is improved during the solidification.
2] Shrinkage will be minimized.
3] Micro shrinkage is less due to fine structure.[ as the fine and staggered dendrites.]
4] Good mechanical property.
5] Less rejection and higher the productivity.
In the grain refinement process it is very essential that to create the highest possible number of active and more fine number of nuclei are require.For this achievement the TI-b alloy is used as,
Ti 5% B
1% Rest is aluminum.
This wire type or rods are solidifying very fast and having more and fine nuclei.
Mixing quantity of the Ti-B
0.1 % Amount is the best combination.
0.05 % is to be added after 2-3 hour in the holding furnace, if there is no any addition of fresh metal in the furnace. If metal remains as it is in the holding furnace more than 4 hours Then fresh treatment is to be given for that material, as the effectiveness of the TiB treatment last only maximum four hours. For the good results and the economic one should use the treated material within four hours.After long time remains in metal nuclei gates coagulated and gets pasivated by other impurities. For this reason the basic alloy should not have Ti content more than 0.1-0.15 % because bigger Titanium nuclei coagulate with the smaller TiB nuclei.
Metal modification
Eutectic point in the AlSi alloy lies at about 12.5 % of the Si. The temperature at this stage lies at 575 degree centigrade. At this temperature the metal liquid metal solidifies in a fine
eutectic structure ,That is fine mixing of aluminum with silicon.
For hypo-eutectic alloy [ Bellow Si =10%] This eutectic structure is surrounded the previous formed aluminum dendrites.
Alloying elements like Na, Sr, and Sb influence the morphology of the eutectic silicon. These elements effluence the following things.
1] Making particles more fine and in sperical shape.
2] Eutectic temperature is lowered by 10 to 15 degree.
3] Eutectic composition is shifted by more than 0.5%.
4] Finer structure improves the mechanical property of the alloy.
Modification with Na.
Modification with Na having the 0.02 to 0.03 % is giving very good results.and mechanical property, in LPDC Die casting, As the Na burns very fast and comes on the surface by
oxidation reaction. The effect of this process last up to one hour. So one should use the treated material within the one hour or give the treatment again.
Modification with Na.
As the treatment given with Na last lesser time,The modification with Sr is used in LPDC
Die casting process. The normal process is done with Sr 0.02 to 0.03 % This modification last more than 4 hours.
Due to this good advantage premodified input alloy is also available.
Benefits of this process.
1] Less rejection % due to porosity in the casting.
2] No leakage in the casting. [ Process can used for pressure tight components.
3] Better mechanical properties.
4] Productivity increases due to no interruption due the casting rejection.
Degassing of the metal.
Degassing operation is very important in LPDC than the pressure die casting. For proper de- gassing rotary degassing unit is preferably to be used.At the same time for oxide removal the flux can be used with the proper feeding arrangement.
The best metal cleaning system is to remove the hydrogen, This is only gas which soluble
in the molten metal and creates gas porasity.H2 gas lowers the density of the metal
creates porosity as well as the fatigue resistance.This type of component then causes leakage under the pressure. It reduces the corrosion resistance, and reduces the tensile and yield strength.Since the solidification rate is lower in the LPDC So H2 porosity formation is more Studying these it is firm that there is greater influence on the mechanical properties of casting.
Also for the micro step cleaning the ceramic filter is used in stacker tube and is also used while tapping the material in the holding furnace.
The method to check the H2 content is to check the sp. Gravity of the metal. The test piece should be solidifies under the low pressure and slowly. The correction can be made as per the results immediately. The std. Can be made for sp. Gravity as per the alloying elements contents in the metal.
It is also clear that the molten metal is absorbing the hydrogen gas from the air. This absorbing is going on after degassing also, This happens when the casting cycle is not in process. After taking out the casting from the mold , the metal is coming contact with the air which is used
for cleaning and air from the surroundings.
In this way the hydrogen level in metal increases. This is depend also on the surrounding air. If there is moisture is the air then H2 pick up is also more.
H2 pick up also more in case where the distance of the mold is more than the level of the molten metal.
If the degassing operation in the transfer ladle, then the time should be as less as possible in between the degassing process and the tapping of the metal.Before that one should see that the mold is ready in all respect to process the castings.Pl. note that degassing is the last
operation of metal treatment.
H2 pick up is going on at the time of pouring the material in the holding furnace, and the after that holding furnace itself. This pick up H2 can not be avoided , but can be checked that the level of H2 cannot go high, tends to have the micro porosity in the casting.
To minimize the H2 in the metal the following care should be taken.
1] Dry air to be used for pressurizing the metal. Using the dry air the level of H2 content is normally constant through the process.
2] In got should be moisture free.
3] Flux also should in dry condition.
Degassing cycle can be set after taking four to five samples from the same melt after constant interval.This can be finalized by taking the specific gravity of the sample and compare the
sp. Gravity. You will get the same specific gravity of the sample after certain level of degassing cycle. And that point should be pick up for degassing cycle time.After that there is unnecessary
of wastage of gas and flux.
Metal composition
Metal composition is very known , because every element got its separate influence in the alloying element.The major effect of mechanical properties of aluminum is influenced by following major elements.
Mg. Content.
Mg has got the major influence on the mechanical properties.in heat treated alloys. Heat treatment only possible when it is maintain in the narrow range, that is below 0.3%
Higher the Mg % Then following results will get.
Elongation is less [Yield strength increases,
Less tough ness after heat treatment.
Less dynamic impact strength.
Hardness and stiffness increases,
Fluidity decreases due to increase in oxidation if Mg at higher side.
Fe content
Following negative points are there when the Fe % is high than specified.
The formation of the needle like phase Al3Fe is harmful. Where it reduces the tensile strength.
It reduces also the elongation, toughness, and dynamic impact strength as the sharp needle points initiates the possible crack. For the safety part the Fe can be used as much low i.s 0.15% But due to the Fe content at higher side the it increases hot strength, so as per the requirement
of property the Fe % can be used up to 1.2% maximum.
Amount more than 1.2% start slugging in the alloy depending upon the Cr, Mn, & Ni present
in the alloy. This slugging starts when the temperature goes down than 650 degree centigrade.
The Fe is increasing due to melting the material with scrap jobs having studs and such iron fittings. Also the wire filter which is used to filter the material.Irrosion of stacker tube and sprue bush If possible ceramic filter as well as tube is to be used to avoid this increasing of iron.
Higher the Fe the casting gets leakage. So for leak proof casting the % of Fe should be controlled as per the requirement of the casting property.
Zn content
Zinc also got the sensitive influence in the alloy.
Zinc causes the hot shrinkage as it is more than 3%
More amount decreases strength and increases the density.
The phenomena of zinc shrinkage is depend upon the component thickness. The shrinkage will start though it is less than 3% in component having more wall thickness.
Cr, and Mn. Content
Small amount I,s less than 0.6% modifies the iron compound to achieve the benefit of iron.
More amount tends to more sludgish.The slugging is directly proportional to the Fe
equivalent. I,s [% Fe + 1.5 x %Mn + 1.2 x % Cr.] should not more than 1.85 max.
Nikel contents.
Nikel is not a serious impurity.But in combination with other heavy elements like iron, chromium and manganese which cause slugging.
Titanium : This is as per the iron. If it is more [0.25%] hard compound may form and tends to reduction in machinability.This is grain refining element, smaller amount of titanium increases
the merchantability. As well as improves the metal structure.
Cadmium. Lead and Tin. : The greater the amount leads to heat cracks and hot shrinkage.
Over 0.1% tin, and 0.05% lead leads to cracking. These elements were very critical and should
be monitored strictly.
Phosprous,and sodium : these promotes drossing.Higher of this increases the H2 pick up and surface tension, which causes the poor fluidity,
Melting and pouring Temperature.
The metal temperature having the important role in the LPDC dies casting. It should not exceed
800 degree centigrade. And the holding furnace temperature should be maintain as 730 degree cent. This is required for.
To reduce the oxidation forming at higher temperature.
To minimize the slag formation.
To have the better heating property and reducing the heat losses.
To save the electricity which is used for melting.
To achieve this the temperature losses should be very very less during the process of transferring the material by transfer ladle, and at the time of pouring. This loss may be 20 degree max. in
transfer ladle, 30 degree while pouring. So the casting temperature should be maintain as 730 d.
Temperature in the holding furnace is to be kept constant in the range of 720 -740 degree. After 75% using of furnace melt, the refilling should be done with Ti B treatment.
Pressure parameter used in LPDC
In LPDC die casting three or four pressure are used. In heavy casting more than that also are used. These pressure are available in new machines.
First phase :
During the first phase metal is to be raised up to the sprue bush only.Prefilling in the first phase should be avoided. This will create the turbulence of the metal. To calculate the First phase pressure P1 the following in put data is required to calculate.
Height of sprue bush bottom to the level of the metal.
Height of the Aluminum column that can be hold against the atmosphere pressure at that place.
For example Hg holds 760 mm at sea level with atmosphere pressure. By this formula one can calculate the pressure require to lift the 1 mm aluminum column. And then P1 can be calculated
by taking the height up to spur bush.
P1 safety pressure can be taken up to the filter height from the metal level.
Filling phase or 2nd phase :
The II nd phase is called as the casting filling phase. This phase slow as possible. The II phase pressure is calculated depending on the casting height plus the pressure head is considered.
The pressure head is considered as maximum 2 times of the casting height.
For example if the casting height is 100 mm then the pressure is calculated as 100mm plus 2x100 Then the final Presser P2 is P1 plus the calculated pressure for the casting height + Head.
Finally P2 is the pressure require to raise the material from its surface to the casting top plus the pressure head for the casting.
III rd Phase
The pressure in this phase is used as much high as possible. This pressure is used to fill the casting with high density at the time of solidification. The maximum pressure is calculated on
the following basis.
1] Die should not flash
2] Sand core should not break and sand should not fuse.
3] Holding furnace should not break.
Holding phase
Most of the times this phase is given in between second and third phase. This holding phase is
holding the pressure which is given at the time of second phase. Then third phase is started
when the material start solidify.In some times the pressure is raised slightly with a long time.
The time for this phase is mostly depend upon the mass of the casting.
Cooling phase
After solidification of the casting the cooling phase is started. This phase is pressure less cycle. The cycle time is precisely calculated and then die opening is started. At this particular time the molten metal should go down with breaking the neck bellow the sprue bush.
At the time of 4 th phase sometimes the outside source of forced air can be used to solidify the heavy sections around the gate and runner. The casting solidifies from the top and the less wall thickness of the casting. The cooling is started mostly after the 3 rd phase. It is adjusted in such
a way that the solidification will end at the sprue bush neck only.
Delta pressure or Correction pressure.
When the casting is completed the mass is reduced in furnace as per the mass of the casting shot. So the every shot the height of the metal level is reducing by some height. For the next casting the extra pressure is required to lift the material from this reduced level .
The extra pressure require after each casting is called as delta pressure or the correction pressure. For this calculation the average diameter of the furnace is taken.
By this correction the filling pressure for the first casting and the last casting remains the same.
Riser tube heating
Riser heating is one of the vital aspect in the LPDC process. When the pressure is released the metal the riser tube is going down, it is breaking at the neck area of the casting. Till the casting solidification it is very important that to maintain thru connector temperature at certain level so that after the solidification of the casting the connecter area should be hot enough to break the connection between casting and the metal coloum in the tube.
To achieve this step the temerature is maintain above 380 degree to 420 degree.
If the connecter area is isolated to prevent the heat transfer, and the temperature is maintained in between the range then there is no need of heating the connecter. In this process one has to reduce the height of the connecter as much as possible.。